Energy storage system utilizing compressed gas

ABSTRACT

An energy storage and recovery system employs air compressed utilizing power from an operating wind turbine. This compressed air is stored within one or more chambers of a structure supporting the wind turbine above the ground. By functioning as both a physical support and as a vessel for storing compressed air, the relative contribution of the support structure to the overall cost of the energy storage and recovery system may be reduced, thereby improving economic realization for the combined turbine/support apparatus. In certain embodiments, expansion forces of the compressed air stored within the chamber, may be relied upon to augment the physical stability of a support structure, further reducing material costs of the support structure.

BACKGROUND

Air compressed to 300 bar has energy density comparable to that oflead-acid batteries and other energy storage technologies. One source ofcompressed air is wind.

It is known that the efficiency of power generation from wind, improveswith increased height of elevation of the fan blades of the wind turbinefrom the ground. Such elevation, however, requires provision of a large,fixed structure of sufficient mechanical strength to safely support therelatively heavy structure of the turbine, including the blades, under avariety of wind conditions.

The expense of constructing and maintaining such a support structure isan inherent expense of the system, detracting from the overallprofitability of the wind generation device. Accordingly, there is aneed in the art for novel structures and methods for supporting a windturbine.

SUMMARY

An energy storage and recovery system employs air compressed utilizingpower from an operating wind turbine. This compressed air is storedwithin one or more chambers of a structure supporting the wind turbineabove the ground. By functioning as both a physical support and as avessel for storing compressed air, the relative contribution of thesupport structure to the overall cost of the energy storage and recoverysystem may be reduced, thereby improving economic realization for thecombined turbine/support apparatus. In certain embodiments, expansionforces of the compressed air stored within the chamber may be reliedupon to augment the physical stability of a support structure, furtherreducing material costs of the support structure.

An embodiment of a method in accordance with the present inventioncomprises storing compressed gas generated from power of an operatingwind turbine, within a chamber defined by walls of a structuresupporting the wind turbine.

An embodiment of an apparatus in accordance with the present inventioncomprises a support structure configured to elevate a wind turbine abovethe ground, the support structure comprising walls defining a chamberconfigured to be in fluid communication with a gas compressor operatedby the wind turbine, the chamber also configured to store gas compressedby the compressor.

An embodiment of an apparatus in accordance with the present inventioncomprises an energy storage system comprising a wind turbine, a gascompressor configured to be operated by the wind turbine, and a supportstructure configured to elevate the wind turbine above the ground, thesupport structure comprising walls defining a chamber in fluidcommunication with the gas compressor, the chamber configured to storegas compressed by the gas compressor. A generator is configured togenerate electrical power from expansion of compressed gas flowed fromthe chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic representation of an embodiment of asystem in accordance with the present invention.

FIG. 1A shows a simplified top view of one embodiment of a planetarygear system which could be used in embodiments of the present invention.FIG. 1AA shows a simplified cross-sectional view of the planetary gearsystem of FIG. 1A taken along line 1A-1A′.

FIG. 2 is a simplified schematic representation of an alternativeembodiment of a system in accordance with the present invention.

FIG. 3 is a simplified schematic representation of an alternativeembodiment of a system in accordance with the present invention.

FIG. 3A is a simplified schematic representation of an alternativeembodiment of a system in accordance with the present invention.

FIG. 4 is a simplified schematic representation of an alternativeembodiment of a system in accordance with the present invention.

FIG. 5 is a simplified schematic representation of an alternativeembodiment of a system in accordance with the present invention.

FIGS. 6-12 show various embodiments. While certain drawings and systemsdepicted herein may be configured using standard symbols, the drawingshave been prepared in a more general manner to reflect the varietyimplementations that may be realized from different embodiments.

DETAILED DESCRIPTION

As previously described, a wind turbine operates to capture wind energymore effectively the higher it is elevated above the ground. Inparticular, wind speed is roughly proportional to the seventh root ofthe height. Power is proportional to the cube of the wind speed, andalso proportional to the area of the wind turbine. A greater height, H,could theoretically allow a larger diameter turbine, giving areaproportional to H² and power proportional to H^(x), with x perhaps asgreat as 2 3/7. The support structure is thus a necessary element of thesystem. According to embodiments of the present invention, this supportstructure can perform the further duty of housing one or more chambersor vessels configured to receive and store compressed air generated fromoutput of the wind turbine.

Such a support structure for a wind turbine is initially well suited forthis task, as it is typically formed from an exterior shell thatencloses an interior space. This structure provides the desiredmechanical support for the wind turbine at the top, while not consumingthe large amount of material and avoiding the heavy weight that wouldotherwise be associated with an entirely solid supporting structure.

FIG. 1 shows a simplified schematic view of an embodiment of a system inaccordance with the present invention. Specifically, system 100comprises a nacelle 101 that is positioned on top of support tower 106.Nacelle 101 includes a wind turbine 102 having rotatable blades 104.

Nacelle 101 may be in rotatable communication (indicated by arrow 120)with support tower 106 through joint 111, thereby allowing the blades ofthe wind turbine to be oriented to face the direction of the prevailingwind. An example of a wind turbine suitable for use in accordance withembodiment of the present invention is the model 1.5 sle turbineavailable from the General Electric Company of Fairfield, Conn.

Upon exposure to wind 108, the blades 104 of the turbine 102 turn,thereby converting the power of the wind into energy that is output onlinkage 105. Linkage 105 may be mechanical, hydraulic, or pneumatic innature.

Linkage 105 is in turn in physical communication with a motor/generator114 through gear system 112 and linkage 103. Gear system 112 is also inphysical communication with compressor/expander element 116 throughlinkage 107. Linkages 103 and 107 may be mechanical, hydraulic, orpneumatic in nature.

The gear system may be configured to permit movement of all linkages atthe same time, in a subtractive or additive manner. The gear system mayalso be configured to accommodate movement of fewer than all of thelinkages. In certain embodiments, a planetary gear system may bewell-suited to perform these tasks.

Compressed gas storage chamber 118 is defined within the walls 118 a ofthe support tower. Compressor/expander 116 is in fluid communicationwith storage chamber 118 through conduit 109.

Several modes of operation of system 100 are now described. In one modeof operation, the wind is blowing, and demand for power on the grid ishigh. Under these conditions, substantially all of the energy outputfrom rotation of the blades of the turbine, is communicated throughlinkages 105 and 103 and gear system 112 to motor/generator 114 that isacting as a generator. Electrical power generated by motor/generator 114is in turn communicated through conduit 113 to be output onto the gridfor consumption. The compressor/expander 116 is not operated in thismode.

In another mode of operation, the wind is blowing but demand for poweris not as high. Under these conditions, a portion of the energy outputfrom rotation of the blades of the turbine is converted into electricalpower through elements 105, 112, 103, and 114 as described above.

Moreover, some portion of the energy output from the operating turbineis also communicated through linkages 105 and 107 and gear system 112 tooperate compressor/expander 116 that is functioning as a compressor.Compressor/expander 116 functions to intake air, compress that air, andthen flow the compressed air into the storage chamber 118 located in thesupport tower. As described below, energy that is stored in the form ofthis compressed air can later be recovered to produce useful work.

Specifically, in another mode of operation of system 100, thecompressor/expander 116 is configured to operate as an expander. In thismode, compressed air from the storage chamber is flowed through conduit109 into the expander 116, where it is allowed to expand. Expansion ofthe air drives a moveable element that is in physical communication withlinkage 107. One example of such a moveable element is a piston that ispositioned within a cylinder of the compressor/expander 116.

The energy of actuated linkage 107 is in turn communicated through gearsystem 112 and linkage 103 to motor/generator 114 that is acting as agenerator. Electrical power generated by motor/generator as a result ofactuation of linkage 103, may in turn be output to the power gridthrough conduit 113.

In the mode of operation just described, the wind may or may not beblowing. If the wind is blowing, the energy output by thecompressor/expander 116 may be combined in the gear system with theenergy output by the turbine 112. The combined energy from these sources(wind, compressed air) may then be communicated by gear system 112through linkage 103 to motor/generator 114.

In still another mode of operation, the wind may not be blowing andpower demand is low. Under these conditions, the compressor/expander 116may operate as a compressor. The motor/generator 114 operates as amotor, drawing power off of the grid to actuate the compressor/expander116 (functioning as a compressor) through linkages 103 and 107 and gearsystem 112. This mode of operation allows excess power from the grid tobe consumed to replenish the compressed air stored in the chamber 118for consumption at a later time.

Embodiments of systems which provide for the efficient storage andrecovery of energy as compressed gas, are described in the U.S.Provisional Patent Application No. 61/221,487 filed Jun. 29, 2009, andin the U.S. nonprovisional patent application Ser. No. 12/695,922 filedJan. 28, 2010, both of which are incorporated by reference in theirentireties herein for all purposes. However, embodiments of the presentinvention are not limited to use with these or any other particulardesigns of compressed air storage and recovery systems. Alsoincorporated by reference in its entirety herein for all purposes, isthe provisional patent application no. 61/294,396, filed Jan. 12, 2010.

As previously mentioned, certain embodiments of the present inventionmay favorably employ a planetary gear system to allow the transfer ofmechanical energy between different elements of the system. Inparticular, such a planetary gear system may offer the flexibility toaccommodate different relative motions between the linkages in thevarious modes of operation described above.

FIG. 1A shows a simplified top view of one embodiment of a planetarygear system which could be used in embodiments of the present invention.FIG. 1AA shows a simplified cross-sectional view of the planetary gearsystem of FIG. 1A taken along line 1A-1A′.

Specifically, planetary gear system 150 comprises a ring gear 152 havinga first set of teeth 154 on an outer periphery, and having a second setof teeth 156 on an inner portion. Ring gear 152 is engaged with, andmoveable in either direction relative to, three other gear assemblies.

In particular, first gear assembly 140 comprises side gear 142 that ispositioned outside of ring gear 152, and is fixed to rotatable shaft 141which serves as a first linkage to the planetary gear system. The teethof side gear 142 are in mechanical communication with the teeth 154located on the outer periphery of the ring gear. Rotation of shaft 141in either direction will translate into a corresponding movement of ringgear 152.

A second gear assembly 158 comprises a central (sun) gear 160 that ispositioned inside of ring gear 152. Central gear 160 is fixed torotatable shaft 162 which serves as a second linkage to the planetarygear system.

Third gear assembly 165 allows central gear 160 to be in mechanicalcommunication with the second set of teeth 156 of ring gear 152. Inparticular, third gear assembly 165 comprises a plurality of (planet)gears 164 that are in free rotational communication through respectivepins 167 with a (planet carrier) plate 166. Plate 166 is fixed to athird shaft 168 serving as a third linkage to the planetary gear system.

The planetary gear system 150 of FIGS. 1A-1AA provides mechanicalcommunication with three rotatable linkages 141, 162, and 168. Each ofthese linkages may be in physical communication with the various otherelements of the system, for example the wind turbine, a generator, amotor, a motor/generator, a compressor, an expander, or acompressor/expander.

The planetary gear system 150 permits movement of all of the linkages atthe same time, in a subtractive or additive manner. For example wherethe wind is blowing, energy from the turbine linkage may be distributedto drive both the linkage to a generator and the linkage to acompressor. In another example, where the wind is blowing and demand forenergy is high, the planetary gear system permits output of the turbinelinkage to be combined with output of an expander linkage, to drive thelinkage to the generator.

Moreover, the planetary gear system is also configured to accommodatemovement of fewer than all of the linkages. For example, rotation ofshaft 141 may result in the rotation of shaft 162 or vice-versa, whereshaft 168 is prevented from rotating. Similarly, rotation of shaft 141may result in the rotation of only shaft 168 and vice-versa, or rotationof shaft 162 may result in the rotation of only shaft 168 andvice-versa. This configuration allows for mechanical energy to beselectively communicated between only two elements of the system, forexample where the wind turbine is stationary and it is desired tooperate a compressor based upon output of a motor.

Returning to FIG. 1, certain embodiments of compressed gas storage andrecovery systems according to the present invention may offer a numberof potentially desirable characteristics. First, the system leveragesequipment that may be present in an existing wind turbine system. Thatis, the compressed air energy storage and recovery system may utilizethe same electrical generator that is used to output power from the windturbine onto the grid. Such use of the generator to generate electricalpower both from the wind and from the stored compressed air, reduces thecost of the overall system.

Another potential benefit associated with the embodiment of FIG. 1 isimproved efficiency of power generation. Specifically, the mechanicalenergy output by the rotating wind turbine blades, is able to becommunicated in mechanical form to the compressor without the need forconversion into another form (such as electrical energy). By utilizingthe output of the power source (the wind turbine) in its nativemechanical form, the efficiency of transfer of that power intocompressed air may be enhanced.

Still another potential benefit associated with the embodiment of FIG. 1is a reduced number of components. In particular, two of the elements ofthe system perform dual functions. Specifically, the motor/generator canoperate as a motor and as a generator, and the compressor/expander canoperate as a compressor or an expander. This eliminates the need forseparate, dedicated elements for performing each of these functions.

Still another potential benefit of the embodiment of FIG. 1 is relativesimplicity of the linkages connecting various elements with movingparts. Specifically, in the embodiment of FIG. 1, the turbine, the gearsystem, the motor/generator, and the compressor/expander are all locatedin the nacelle. Such a configuration offers the benefit of compatibilitywith a rotational connection between a nacelle and the underlyingsupport structure. In particular, none of the linkages between theelements needs to traverse the rotating joint, and thus the linkages donot need to accommodate relative motion between the nacelle and supportstructure. Such a configuration allows the design and operation of thoselinkages to be substantially simplified.

According to alternative embodiments, however, one or more of the gearsystem, the compressor/expander, and the motor/generator may bepositioned outside of the nacelle. FIG. 2 shows a simplified view ofsuch an alternative embodiment of a system 200 in accordance with thepresent invention.

In this embodiment, while the turbine 202 is positioned in the nacelle201, the gear system 212, compressor/expander 216, and motor generator214 are located at the base of the tower 206. This placement is madepossible by the use of an elongated linkage 205 running between turbine202 and gear system 212. Elongated linkage 205 may be mechanical,hydraulic, or pneumatic in nature.

The design of the embodiment of FIG. 2 may offer some additionalcomplexity, in that the linkage 205 traverses rotating joint 211 andaccordingly must be able to accommodate relative motion of the turbine202 relative to the gear system 212. Some of this complexity may bereduced by considering that linkage 205 is limited to communicatingenergy in only one direction (from the turbine to the gear system).

Moreover, the cost of complexity associated with having linkage 205traverse rotating joint 211, may be offset by the ease of access to themotor/generator, compressor/expander, and gear system. Specifically,these elements include a large number of moving parts and are subject towear. Positioning these elements at the base of the tower (rather thanat the top) facilitates access for purposes of inspection andmaintenance, thereby reducing cost.

Still other embodiments are possible. For example, while FIG. 2 showsthe gear system, motor/generator, and compressor/expander elements asbeing housed within the support structure, this is not required. Inother embodiments, one or more of these elements could be locatedoutside of the support structure, and still communicate with the windturbine through a linkage extending from the support tower. In suchembodiments, conduits for compressed air and for electricity, andmechanical, hydraulic, or pneumatic linkages could provide for thenecessary communication between system elements.

Embodiments of the present invention are not limited to the particularelements described above. For example, while FIGS. 1 and 2 showcompressed gas storage system comprising compressor/expander elementsand motor/generator elements having combined functionality, this is notrequired by the present invention.

FIG. 3 shows an alternative embodiment a system 300 according to thepresent invention, utilizing separate, dedicated compressor 350,dedicated expander 316, dedicated motor 354, and dedicated generator 314elements. Such an embodiment may be useful to adapt an existing windturbine to accommodate a compressed gas storage system.

Specifically, pre-existing packages for wind turbines may feature thededicated generator element 314 in communication with the turbine 302through gear system 312 and linkages 303 and 305. Generator 314,however, is not designed to also exhibit functionality as a motor.

To such an existing configuration, a dedicated expander 316, a dedicatedcompressor 350, a dedicated motor 354, linkages 307 and 373, and conduit370 may be added to incorporate a compressed gas storage system. In oneembodiment, a dedicated expander 316 may be positioned in the nacelle301 in communication with the gear system 312 through linkage 307.Dedicated expander 316 is in fluid communication with a top portion ofthe compressed gas storage chamber 318 defined within the walls 306 a ofsupport tower 306 through conduit 309.

Dedicated compressor 350 and a dedicated motor 354 are readily included,for example at or near the base of the support tower, therebyfacilitating access to these elements. Dedicated compressor 350 is influid communication with storage chamber 318 through conduit 370, and inphysical communication with dedicated motor 354 through linkage 372.Dedicated motor 354 is in turn in electronic communication with thegenerator and/or grid to receive power to operate the compressor toreplenish the supply of compressed gas stored in the chamber 318.

As shown in FIG. 3, this embodiment may further include an optionalelongated mechanical, hydraulic, or pneumatic linkage 374 extendingbetween the gear system 312 in the nacelle 301, and the dedicatedcompressor 350 located outside of the nacelle 301. Such a linkage wouldallow the dedicated compressor to be directly operated by the output ofthe turbine, avoiding losses associated with converting mechanical intoelectrical form by the dedicated generator, and re-converting theelectrical power back into mechanical form by the dedicated motor inorder to operate the compressor.

FIG. 3A shows a simplified view of yet another embodiment of a system inaccordance with the present invention. In the embodiment of the system380 of FIG. 3A, only the turbine 382, linkage 383, and dedicatedcompressor 386 elements are located in the nacelle 381 that ispositioned atop support tower 396. Dedicated compressor 386 is incommunication with the turbine through linkage 383 (which may bemechanical, hydraulic, or pneumatic), which serves to drive compressionof air by the dedicated compressor. Compressed air output by thededicated compressor is flowed through conduit 389 across joint 391 intochamber 398 present in the support tower 396.

The remaining elements are positioned outside of the nacelle, either inthe support tower, or alternatively outside of the support tower. Forexample, a dedicated expander or expander/compressor 388 is incommunication with the chamber 398 defined within walls 396 a, toreceive compressed air through conduit 393. Element 388 is configured toallow expansion of the compressed air, and to communicate energyrecovered from this expansion through linkage 392 to generator orgenerator/motor 384. Element 384 in turn operates to generateelectricity that is fed onto the grid.

The embodiment of FIG. 3A can also function to store energy off of thegrid. Where element 384 is a generator/motor and element 388 is anexpander/compressor, element 384 may operate as a motor to drive element388 operating as a compressor, such that air is compressed and flowedinto chamber 398 for storage and later recovery.

The embodiment of FIG. 3A offers a potential advantage in that power istransported from the top to the bottom of the tower utilizing thechamber, without requiring a separate elongated linkage or conduit.Another possible advantage of the embodiment of FIG. 3A is a reductionin the weight at the top of the tower. While this embodiment may incurlosses where the mechanical power output of the turbine is convertedfirst into compressed air and then back into mechanical power fordriving the generator, such losses may be offset by a reduction inweight at the top of the tower, allowing the tower to be higher and toaccess more wind power.

The present invention is not limited to a support structure having anyparticular shape. In the particular embodiments shown in FIGS. 1 and 2,the support structure exhibits a cross-sectional shape that varies alongits length. For example, the support structure 106 is wide at its base,and then tapers to a point at which it meets the wind turbine. Byallocating material to where it will best serve the supporting function,such a design minimizes materials and reduces cost.

However, the present invention also encompasses supporting structureshaving other shapes. For example, FIG. 4 shows a support structure 400comprising a hollow tube having a circular or elliptical cross sectionthat is substantially uniform. The walls 400 a of this hollow tube 400in turn define a chamber 402 for storing compressed gas. While possiblyutilizing more mass, such a tube is a simpler structure that is employedfor a various applications in many other industries. Accordingly, such atube is likely available at a relatively low price that may offset anygreater material cost.

Still further alternative embodiments are possible. For example, incertain embodiments a support structure may be designed to takeadvantage of the forces exerted by the compressed air stored therein, inorder to impart additional stability to the support structure.

Thus, FIG. 5 shows an embodiment wherein the support structure 500comprises a portion 506 a having thinner walls 506 b exhibiting lessinherent strength than those of the prior embodiments. This reducedstrength may be attributable to one or more factors, including but notlimited to, use of a different design or shape for the support, use of areduced amount of material in the support, or use of a differentmaterial in the support.

According to embodiments of the present invention, however, anyreduction in the inherent strength of the support structure 506 may beoffset by expansion forces 524 exerted by the compressed air 526 that iscontained within the chamber 518. Specifically, in a manner analogous tothe stiffening of walls of an inflated balloon, the expansion force ofthe compressed air may contribute additional strength to the supportstructure. This expansion effect is shown grossly exaggerated in FIG. 5,for purposes of illustration.

One possible application for such a design, employs a support structurethat is fabricated from a material that is capable of at least someflexion, for example carbon fiber. In such an embodiment, expansionforces from the compressed air within the chamber of a flexible supportmember, may act against the walls of the chamber, thereby stiffening itand contributing to the structural stability of that support. Such asupport structure could alternatively be formed from other materials,and remain within the scope of the present invention.

A design incorporating carbon fiber could offer even further advantages.For example, carbon fiber structures may exhibit enhanced strength inparticular dimensions, depending upon the manner of their fabrication.Thus, a carbon fiber support structure could be fabricated to exhibitstrength and/or flexion in particular dimensions, for example those inwhich the expansion forces of the compressed air are expected tooperate, and/or dimension in which the support is expected to experienceexternal stress (e.g. a prevailing wind direction).

Of course, a design taking advantage of expansion forces of the storedcompressed air, would need to exhibit sufficient inherent strength inthe face of expected (and unexpected) changes in the quantity ofcompressed air stored therein, as that compressed air is drawn away andallowed to expand for energy recovery. Nevertheless, expansion forcesassociated with minimal amounts of compressed air remaining within thesupport structure, could impart sufficient stability to supportstructure to reduce its cost of manufacture and maintenance.

Embodiments of the present invention relate generally to energy storagesystems, and more particularly, relates to energy storage systems thatutilize compressed air as the energy storage medium, comprising an aircompression/expansion mechanism, a heat exchanger, and one or more airstorage tanks.

According to embodiments of the present invention, a compressed-airenergy storage system is provided comprising a reversible mechanism tocompress and expand air, one or more compressed air storage tanks, acontrol system, one or more heat exchangers, and, in certain embodimentsof the invention, a motor-generator (for example motor-generator 697 ofFIG. 6).

The reversible air compressor-expander uses mechanical power to compressair (when it is acting as a compressor) and converts the energy storedin compressed air to mechanical power (when it is acting as anexpander). The compressor-expander comprises one or more stages, eachstage consisting of pressure vessel (the “pressure cell”) partiallyfilled with water or other liquid. In some embodiments, the pressurevessel communicates with one or more cylinder devices to exchange airand liquid with the cylinder chamber(s) thereof. Suitable valving allowsair to enter and leave the pressure cell and cylinder device, ifpresent, under electronic control.

The cylinder device referred to above may be constructed in one ofseveral ways. In one specific embodiment, it can have a piston connectedto a piston rod, so that mechanical power coming in or out of thecylinder device is transmitted by this piston rod. In anotherconfiguration, the cylinder device can contain hydraulic liquid, inwhich case the liquid is driven by the pressure of the expanding air,transmitting power out of the cylinder device in that way. In such aconfiguration, the hydraulic liquid can interact with the air directly,or a diaphragm across the diameter of the cylinder device can separatethe air from the liquid.

In low-pressure stages, liquid is pumped through an atomizing nozzleinto the pressure cell or, in certain embodiments, the cylinder deviceduring the expansion or compression stroke to facilitate heat exchange.The amount of liquid entering the chamber is sufficient to absorb(during compression) or release (during expansion) all the heatassociated with the compression or expansion process, allowing thoseprocesses to proceed near-isothermally. This liquid is then returned tothe pressure cell during the non-power phase of the stroke, where it canexchange heat with the external environment via a conventional heatexchanger. This allows the compression or expansion to occur at highefficiency.

Operation of embodiments according the present invention may becharacterized by a magnitude of temperature change of the gas beingcompressed or expanded. According to one embodiment, during acompression cycle the gas may experience an increase in temperate of 100degrees Celsius or less, or a temperature increase of 60 degrees Celsiusor less. In some embodiments, during an expansion cycle, the gas mayexperience a decrease in temperature of 100 degrees Celsius or less, 15degrees Celsius or less, or 11 degrees Celsius or less—nearing thefreezing point of water from an initial point of room temperature.

Instead of injecting liquid via a nozzle, as described above, air may bebubbled though a quantity of liquid in one or more of the cylinderdevices in order to facilitate heat exchange. This approach is preferredat high pressures.

During expansion, the valve timing is controlled electronically so thatonly so much air as is required to expand by the desired expansion ratiois admitted to the cylinder device. This volume changes as the storagetank depletes, so that the valve timing must be adjusted dynamically.

The volume of the cylinder chambers (if present) and pressure cellsincreases from the high to low pressure stages. In other specificembodiments of the invention, rather than having cylinder chambers ofdifferent volumes, a plurality of cylinder devices is provided withchambers of the same volume are used, their total volume equating to therequired larger volume.

During compression, a motor or other source of shaft torque drives thepistons or creates the hydraulic pressure via a pump which compressesthe air in the cylinder device. During expansion, the reverse is true.Expanding air drives the piston or hydraulic liquid, sending mechanicalpower out of the system. This mechanical power can be converted to orfrom electrical power using a conventional motor-generator.

While the present invention will be described with reference to a fewspecific embodiments, the description is illustrative of the inventionand is not to be construed as limiting the invention. Variousmodifications to the present invention can be made to the preferredembodiments by those skilled in the art without departing from the truespirit and scope of the invention. It will be noted here that for abetter understanding, like components are designated by like referencenumerals throughout the various figures.

Single-Stage System

FIG. 6 depicts the simplest embodiment of the compressed air energystorage system 20 of the present invention, and illustrates many of theimportant principles. Briefly, some of these principles which improveupon current compressed air energy storage system designs include mixinga liquid with the air to facilitate heat exchange during compression andexpansion, thereby improving the efficiency of the process, and applyingthe same mechanism for both compressing and expanding air. Lastly, bycontrolling the valve timing electronically, the highest possible workoutput from a given volume of compressed air can be obtained.

As best shown in FIG. 6, the energy storage system 620 includes acylinder device 621 defining a chamber 622 formed for reciprocatingreceipt of a piston device 623 or the like therein. The compressed airenergy storage system 620 also includes a pressure cell 625 which whentaken together with the cylinder device 621, as a unit, form a one stagereversible compression/expansion mechanism (i.e., a one-stage 624).There is an air filter 626, a liquid-air separator 627, and a liquidtank 628, containing a liquid 649 d fluidly connected to thecompression/expansion mechanism 624 on the low pressure side via pipes630 and 631, respectively. On the high pressure side, an air storagetank or tanks 632 is connected to the pressure cell 625 via input pipe633 and output pipe 634. A plurality of two-way, two position valves635-643 are provided, along with two output nozzles 611 and 644. Thisparticular embodiment also includes liquid pumps 646 and 647. It will beappreciated, however, that if the elevation of the liquid tank 628 ishigher than that of the cylinder device 621, water will feed into thecylinder device by gravity, eliminating the need for pump 646.

Briefly, atmospheric air enters the system via pipe 610, passes throughthe filter 626 and enters the cylinder chamber 622 of cylinder device621, via pipe 630, where it is compressed by the action of piston 623,by hydraulic pressure, or by other mechanical approaches (see FIG. 11).Before compression begins, a liquid mist is introduced into the chamber622 of the cylinder device 621 using an atomizing nozzle 644, via pipe648 from the pressure cell 625. This liquid may be water, oil, or anyappropriate liquid 649 f from the pressure cell having sufficient highheat capacity properties. The system preferably operates atsubstantially ambient temperature, so that liquids capable ofwithstanding high temperatures are not required. The primary function ofthe liquid mist is to absorb the heat generated during compression ofthe air in the cylinder chamber. The predetermined quantity of mistinjected into the chamber during each compression stroke, thus, is thatrequired to absorb all the heat generated during that stroke. As themist condenses, it collects as a body of liquid 649 e in the cylinderchamber 622.

The compressed air/liquid mixture is then transferred into the pressurecell 625 through outlet nozzle 611, via pipe 651. In the pressure cell625, the transferred mixture exchanges the captured heat generated bycompression to a body of liquid 649 f contained in the cell. The airbubbles up through the liquid and on to the top of the pressure cell,and then proceeds to the air storage tank 632, via pipe 633.

The expansion cycle is essentially the reverse process of thecompression cycle. Air leaves the air storage tank 632, via pipe 634,bubbling up through the liquid 649 f in the pressure cell 625, entersthe chamber 622 of cylinder device 621, via pipe 655, where it drivespiston 623 or other mechanical linkage. Once again, liquid mist isintroduced into the cylinder chamber 622, via outlet nozzle 644 and pipe648, during expansion to keep a substantially constant temperature inthe cylinder chamber during the expansion process. When the airexpansion is complete, the spent air and mist pass through an air-liquidseparator 627 so that the separated liquid can be reused. Finally, theair is exhausted to the atmosphere via pipe 610.

The liquid 649 f contained in the pressure cell 625 is continuallycirculated through the heat exchanger 652 to remove the heat generatedduring compression or to add the heat to the chamber to be absorbedduring expansion. This circulating liquid in turn exchanges heat with athermal reservoir external to the system (e.g. the atmosphere, a pond,etc.) via a conventional air or water-cooled heat exchanger (not shownin this figure). The circulating liquid is conveyed to and from thatexternal heat exchanger via pipes 653 and 654 communicating withinternal heat exchanger 652.

The apparatus of FIG. 6 further includes a controller/processor 6004 inelectronic communication with a computer-readable storage device 6002,which may be of any design, including but not limited to those based onsemiconductor principles, or magnetic or optical storage principles.Controller 6004 is shown as being in electronic communication with auniverse of active elements in the system, including but not limited tovalves, pumps, chambers, nozzles, and sensors. Specific examples ofsensors utilized by the system include but are not limited to pressuresensors (P) 6008, 6014, and 6024, temperature sensors (T) 6010, 6018,6016, and 6026, humidity sensor (H) 6006, volume sensors (V) 6012 and6022, and flow rate sensor 6020.

As described in detail below, based upon input received from one or moresystem elements, and also possibly values calculated from those inputs,controller/processor 6004 may dynamically control operation of thesystem to achieve one or more objectives, including but not limited tomaximized or controlled efficiency of conversion of stored energy intouseful work; maximized, minimized, or controlled power output anexpected power output an expected output speed of a rotating shaft incommunication with the piston; an expected output torque of a rotatingshaft in communication with the piston; an expected input speed of arotating shaft in communication with the piston; an expected inputtorque of a rotating shaft in communication with the piston; a maximumoutput speed of a rotating shaft in communication with the piston; amaximum output torque of a rotating shaft in communication with thepiston; a minimum output speed of a rotating shaft in communication withthe piston; a minimum output torque of a rotating shaft in communicationwith the piston; a maximum input speed of a rotating shaft incommunication with the piston; a maximum input torque of a rotatingshaft in communication with the piston; a minimum input speed of arotating shaft in communication with the piston; a minimum input torqueof a rotating shaft in communication with the piston; or a maximumexpected temperature difference of air at each stage.

The compression cycle for this single-stage system proceeds as follows:

Step 1 2 3 4 5 Description Move Add liquid Add mist compressed Refill tocylinder to cylinder air to cylinder device device Compress pressurecell device Valve 635 Open Closed Closed Closed Closed Valve 636 OpenClosed Closed Closed Open Valve 637 Closed Closed Closed Closed ClosedValve 638 Closed Closed Closed Open Closed Valve 639 Closed Open ClosedClosed Closed Valve 640 Closed Closed Closed Closed Closed Valve 641Closed Closed Closed Open Closed Valve 642 Open Closed Closed ClosedClosed Valve 643 Closed Closed Closed Closed Open Pump 646 On Off OffOff Off Pump 647 Off On Off Off Off Piston 623 Near bottom Near BDC AtBDC Between At TDC dead at start BDC and at start center (BDC) of stepTDC of step

During step 1 of the compression cycle, liquid 649 d is added to thechamber 622 of the cylinder device 621 from the liquid tank 628(collecting as body of liquid 649 e) such that, when the piston 623reaches top dead center (TDC), the dead volume in the cylinder device iszero. This will only have to be done occasionally, so that this step isomitted on the great majority of cycles.

During step 2 of the compression cycle, liquid mist from pressure cell625 is pumped, via pump 647, into the cylinder chamber 622, via pipe 648and nozzle 644. The selected quantity of mist is sufficient to absorbthe heat generated during the compression step (step 3). The volumefraction of liquid must sufficiently low enough that the droplets willnot substantially fuse together, thus reducing the effective surfacearea available for heat exchange that is, the interface between air andliquid). Typically, the pressure differential between the pressure cell625 and the chamber 622 of the cylinder device 621 is sufficiently highso that the operation of pump 647 is not required.

During step 3 of the compression cycle, the piston 623 is driven upwardby a crankshaft 699 coupled to a piston rod 619, by hydraulic pressure,or by some other mechanical structure, compressing the air and mistcontained in the cylinder chamber.

Step 4 of the compression cycle begins when the air pressure inside thecylinder chamber 622 is substantially equal to the pressure inside thepressure cell 625, at which point outlet valve 638 opens, allowingcompressed air to flow from the cylinder chamber to the pressure cell.Because of the liquid added to the cylinder device during step 1 of thecompression cycle, substantially all the air in the cylinder chamber canbe pushed out during this step. The compressed air is introduced intothe pressure cell 625 through an inlet nozzle 611, along with anyentrained mist, creating fine bubbles so that the heat generated duringcompression will exchange with the liquid 649 f in the cell rapidly.

During step 5 of the compression cycle, the piston 623 is pulled downallowing low-pressure air to refill it, via valve 636 and pipe 630. Theabove table shows valve 639 as being closed during this step, and showspump 647 as being off during this step 5. However, this is not required.In other embodiments valve 639 could be open and pump 647 could be on,during the step 5 such that mist is introduced into the cylinder chamberas it is refilled with air.

The expansion cycle for this single-stage system proceeds as follows:

Step 1 2 3 4 Description Add compressed Add liquid air and liquid tocylinder mist to cylinder Exhaust device device Expansion spent airValve 635 Open Closed Closed Closed Valve 636 Open Closed Closed OpenValve 637 Closed Open Closed Closed Valve 638 Closed Closed ClosedClosed Valve 639 Closed Open Closed Closed Valve 640 Closed Open ClosedClosed Valve 641 Closed Closed Closed Closed Valve 642 Closed ClosedClosed Open Valve 643 Closed Closed Closed Closed Pump 646 On Off OffOff Pump 647 Off On Off Off Piston 623 Near TDC At TDC at start Near TDCat At BDC at of step start of step start of step

During step 1 of the expansion cycle, liquid is added to the cylinderchamber from the liquid tank 628 to eliminate dead volume in the system.This will be required only rarely, as mentioned above. Similar to thecompression cycle, the pump 646 can be eliminated if the liquid tank 628is oriented at an elevation higher than that of the chamber of cylinderdevice 621.

During step 2 of the expansion cycle, a pre-determined amount of air,V₀, is added to the chamber of the cylinder device by opening inletvalve 637 for the correct interval, which is dependent on the pressureof the air in the pressure cell and the desired expansion ratio. The V₀required is the total cylinder device volume divided by the desiredexpansion ratio. For a single stage system, that ratio is less than orequal to the pressure of air in the air storage tank in atmospheres. Atthe same time air is being introduced into the cylinder chamber 622,liquid mist from the pressure cell is being pumped (via pump 647)through inlet nozzle 644 into the cylinder chamber. If a sufficientpressure differential exists between the pressure cell 625 and thecylinder device 621, pump 647 is not required. Once the pressure insideof the cylinder chamber is sufficiently high, valve 637 is closed. Thepiston 623 is urged in the direction of BDC beginning with this step,transmitting power out of the system via a crankshaft, hydraulicpressure, or other mechanical structure.

During step 3 of the expansion cycle, the air introduced in step 2 isallowed to expand in the chamber 622. Liquid mist also continues to bepumped into the chamber 622 through nozzle 644. The predetermined totalamount of mist introduced is that required to add enough heat to thesystem to keep the temperature substantially constant during airexpansion. The piston 623 is driven to the bottom of the cylinder deviceduring this step.

It will be appreciated that this two-step expansion process (a quantityof air V₀ introduced in the first step—step 2—and then allowed to expandin the second step—step 3) allows the system to extract substantiallyall the energy available in the compressed air.

During step 4 of the expansion cycle, the crankshaft or other mechanicallinkage moves the piston 619 back up to top dead-center (TDC),exhausting the spent air and liquid mist from the cylinder device. Thepower required to drive the piston comes from the momentum of the systemand/or from the motion of other out-of-phase pistons. The exhausted airpasses through an air-liquid separator, and the liquid that is separatedout is returned to the liquid tank 628.

Options for Conveying Mechanical Power to and from the System

At least four methods may be applied to convey power to and from a stagein accordance with the present invention. These are described asfollows, and illustrated in FIG. 11.

W. A direct-acting hydraulic cylinder device 1121 w is shown andoperates as follows. During the expansion cycle, air entering thechamber 1122 w of cylinder device 1121 w, via valve 1121 w and pipe 1122w, urges the hydraulic liquid 1149 w out through valve 1123 w. It thenflows through pipe 1124 w. The force thus pneumatically applied againstthe liquid can be used to operate a hydraulic device (e.g., a hydraulicmotor, a hydraulic cylinder device or a hydro turbine) to createmechanical power. During the compression cycle, the reverse processoccurs. An external source of mechanical power operates a hydraulic pumpor cylinder device, which forces hydraulic liquid 1149 w into thecylinder chamber 1122 w, through valve 1123 w, compressing the air inthe chamber. When the air has reached the desired pressure, valve 1121 wis opened, allowing the compressed air to flow from the cylinder chamber1122 w to the next higher-pressure stage or to the air tank.

X. A single-acting piston 1123 x may be connected to a conventionalcrankshaft via a piston rod 1119 x. Its operation is described in detailin the section titled Single-Stage System above.

Y. A double-acting piston may similarly be connected to a crankshaft viaa piston rod 1119 y.

Z. A hydraulic cylinder device 1121 with a diaphragm 1125 is illustratedsuch that when air enters the cylinder chamber 1122 z, via valve 1121 z,during the expansion cycle, the diaphragm 1125 is forced downwardly.Consequently, the hydraulic liquid 1149 z is urged or driven throughvalve 1123 z and through pipe 1124 z. Similarly, during compression, thehydraulic liquid 1149 z is driven through valve 1123 z and into thecylinder chamber 1122 z, deflecting the diaphragm 1125 upwardly,compressing the air in the upper part of the chamber 1122 z, which thenexits via valve 1121 z.

Note that all four of these options can be used with either the liquidmist technique or the bubbles technique to effect heat transfer. Thenecessary valves and nozzles to supply the mist or bubbles are not shownon FIG. 11.

While the above examples describe the use of pistons, other types ofmoveable elements may be utilized and still remain within the scope ofthe present invention. Examples of alternative types of apparatuseswhich could be utilized include but are not limited to screwcompressors, multi-lobe blowers, vane compressors, gerotors, andquasi-turbines.

Single-Stage, Single-Acting Enemy Storage System:

Referring now to the embodiment of FIG. 12, a single-stage,single-acting energy storage system 1220 is illustrated that utilizestwo pressure cells 1225 d and 1225 e configured as direct-actinghydraulic cylinder devices (option A above). The two pressure cellsoperate substantially 180 degrees out of phase with each other. Liquidmist is used to effect heat exchange during the compression cycle, andboth bubbles and mist are used to effect heat exchange during theexpansion cycle.

As described above in connection with FIG. 6, the apparatus of FIG. 12further includes a controller/processor 1206 in electronic communicationwith a computer-readable storage device 1208, which may be of anydesign, including but not limited to those based on semiconductorprinciples, or magnetic or optical storage principles. Controller 1206is shown as being in electronic communication with a universe of activeelements in the system, including but not limited to valves, pumps,chambers, nozzles, and sensors. Specific examples of sensors utilized bythe system include but are not limited to pressure sensors (P) 1216,1282, and 1288, temperature sensors (T) 1212, 1218, 1284, 1286, and1287, humidity sensor (H) 1290, and volume sensors (V) 1236, 1214, and1280.

The compression cycle of the single-stage, single-acting energy storagesystem 20 proceeds as follows:

Step 1 2 3 4 Description Compress air in Move com- Compress air in Movecom- cell 1225d while pressed cell 1225e while pressed spraying mist,air from spraying mist, air from and replenish the cell 1225d andreplenish the cell 1225e air in cell 1225e to air tank air in cell 1225dto air tank Valve Closed Closed Open Open 1230 Valve Open Open ClosedClosed 1231 Valve Closed Open Closed Closed 1232 Valve Closed ClosedClosed Closed 1233 Valve Open Open Closed Closed 1234 Valve ClosedClosed Open Open 1235 Valve Closed Closed Closed Open 1236 Valve ClosedClosed Closed Closed 1237 Valve Pump out to cell Pump out Pump out tocell Pump out 1238 1225d, pump in to cell 1225e, pump in to cell fromcell 1225e 1225d, from cell 1225d 1225e, pump pump in from in from cell1225e cell 1225d Pump 46 On On On On

During step 1, fluid is pumped from pressure cell 1225 e using thehydraulic pump-motor 1257 into pressure cell 1225 d, thereby compressingthe air inside cell 1225 d. Fluid mist is sprayed through nozzle 1241,which absorbs the heat of compression. When the pressure inside cell1225 d has reached the pressure of the air tank 1232, valve 1232 isopened to let the compressed air move to the air tank. As these stepshave been progressing, air at atmospheric pressure has entered thesystem via pipe 1210 and air filter 1226 d and thence into cell 1225 eto replace the fluid pumped out of it.

When all the air has been driven out of cell 1225 d, the processreverses, and step 3 commences, with the four-way valve 1238 changingstate to cause liquid to be pumped out of cell 1225 d and into cell 1225e, causing the air in cell 1225 e to be compressed. Thus, liquid ispumped back and forth between cells 1225 d and 1225 e in a continuouscycle.

The expansion cycle of the single-stage, single-acting energy storagesystem proceeds as follows:

In step 1, compressed air is bubbled into pressure cell 1225 d vianozzle 1211 d. As the bubbles rise, they exchange heat with the body offluid 1249 d. Air is forced out of cell 1225 d, passing through pipe1239 d, and then driving hydraulic motor 1257, thereby deliveringmechanical power.

In step 2, the valve 1233 admitting the compressed air into cell 1225 dis closed, allowing the air in cell 1225 d to expand, continuing tooperate motor 1257. In step 3, once the air admitted in step 1 has risento the top of cell 1225 d and can no longer exchange heat with the bodyof fluid 1249 d, fluid mist is sprayed into the cell via nozzle 1241 tofurther warm the expanding air.

As fluid passes through the hydraulic motor 1257 during steps 1, 2, and3, it continues through pipe 1239 e and enters pressure cell 1225 e,urging the air present in that cell through pipe 1240 and into theliquid trap-reservoir 1213 d, and thence into the atmosphere via airfilter 1226 d and finally pipe 1210.

Steps 4, 5, and 6 mirror steps 1, 2, and 3. That is, compressed air isbubbled into pressure cell 1225 e, forcing fluid through the hydraulicmotor 1257, and then into pressure cell 1225 d.

If reservoir 1213 e is depleted during operation, excess liquid ispumped from the bottom of reservoir 1213 d into cells 1225 d and 1225 e,using a pump, not shown in the figure, connected to pipe 1240.

Over time, both liquid traps 1213 d and 1213 e will change temperaturedue to the air and entrained droplets transferring heat—a heatexchanger, as shown by coils 1252 d and 1252 e, in pressure cells 1225 dand 1225 e, and connected to a conventional external heat exchanger 1212that exchanges heat with the environment, will moderate the temperatureto near ambient.

The volume of compressed air bubbled into the cells during steps 1 and 3depends on the power output desired. If the air can expand fully to oneatmosphere without displacing all the liquid in the cell, then themaximum amount of work will be done during the stroke. If the air doesnot fully expand during the stroke, all else being equal the poweroutput will be higher at the expense of efficiency.

Note that the pressure cells cannot be of insufficient height so thatthe air bubbles reach the surface of the liquid during the course of thestroke, since almost all heat exchange with the body of liquid occurswhile the bubbles are rising through it. However, they must besufficiently tall for the column of bubbles to completely separate fromthe fluid by the time the exhaust stroke completes. If the system mustbe run slowly, some of the bubbles will reach the top before expansioncompletes. In this event, liquid mist is sprayed through nozzles 1241(in step 3) or 1242 (in step 6) of the expansion cycle.

Note that all the configurations described herein use and generate powerin mechanical form, be it hydraulic pressure or the reciprocating actionof a piston. In most applications, however, the requirement will be forthe storage of electrical energy. In that case, a generator, along withappropriate power conditioning electronics, must be added to convert themechanical power supplied by the system during expansion to electricalpower. Similarly, the mechanical power required by the system duringcompression must be supplied by a motor. Since compression and expansionare never done simultaneously, a motor-generator may be used to performboth functions. If the energy storage system utilizes a hydraulic motoror a hydro turbine (for example hydraulic motor 1257 of FIG. 12), thenthe shaft 1299 of that device connects directly or via a gearbox 1204 tothe motor-generator 1202. If the energy storage system utilizesreciprocating pistons, then a crankshaft or other mechanical linkagethat can convert reciprocating motion to shaft torque is required.

Use of Waste Heat During Expansion

In order to operate isothermally, the tendency of air to cool as itexpands while doing work (i.e. by pushing a piston or displacinghydraulic liquid) must be counteracted by heat exchange with the ambientair or with a body of water (e.g. a stream or lake). If, however, someother source of heat is available—for example, hot water from a steamcondenser—it may be used advantageously during the expansion cycle. InFIG. 6, as described in the Single-Stage System section above, pipes 653and 654 lead to an external heat exchanger. If those pipes are routedinstead to a heat source, the efficiency of the expansion process can beincreased dramatically.

Because the system operates substantially at or near ambienttemperature, the source of heat need only be a few degrees above ambientin order to be useful in this regard. The heat source must, however,have sufficient thermal mass to supply all the heat required to keep theexpansion process at or above ambient temperature throughout the cycle.

According to certain embodiments, a temperature in the form of heat froma heat source, may be harnessed to generate useable energy fromexpansion of a compressed gas. A compressor-expander is in fluidcommunication with a compressed gas storage unit. Compressed gasreceived from the storage unit, expands in the compressor-expander togenerate power. During expansion, the heat source is in selectivethermal communication with the compressor-expander through a heatexchanger, to enhance power output. System operation may be furtherenhanced by introducing a fluid during expansion, and/or by controllingair flowed into and out of the compressor-expander during expansion.

In order to operate nearly isothermally, the tendency of gas to cool asit expands while doing work (i.e. by pushing a piston or displacinghydraulic liquid), can be counteracted by heat exchange with a heatsource. If some form of heat is available, it may be harnessed toimprove power output during an expansion cycle.

Because in many embodiments a compressed gas system is configured tooperate substantially at or near ambient temperature, the source of heatneed only be a few degrees above ambient in order to be useful in thisregard. The heat source must, however, have sufficient thermal mass tosupply all the heat required to keep the expansion process near ambienttemperature throughout the cycle. Thus, embodiments of the presentinvention may be able to harness low grade heat, for example in the formof waste heat from another process, to enhance the power output fromcompressed air.

FIG. 7 shows a simplified block diagram of an embodiment of a system 780according to the present invention, for generating energy fromcompressed air, although other forms of compressed gas could be used.The system includes a compressor-expander 782 which may have a structuresimilar to that described in U.S. provisional patent application No.61/221,487 (“the '487 application”), but alternatively could be ofanother design.

Compressor-expander 782 is in fluid communication with compressed airstorage unit 784. Compressor-expander 782 is in selective thermalcommunication through heat exchanger 786 and valve 788, with either heatsource 790 or heat sink 792. Heat source 790 may be a source of lowgrade heat or high grade heat. Heat source 790 may be presentcontinuously, or may be intermittent in nature.

Compressor-expander 782 is in physical communication withmotor-generator 794 through linkage 796. Linkage 796 may be mechanical,hydraulic, or pneumatic, depending upon the particular embodiment.Motor-generator 794 is in turn in electrical communication with a powersource such as the electrical grid 798.

Operation of the system 780 is described as follows. In a first mode,system 780 is configured to generate power by converting compressed airstored in the storage unit 784, into useable work. The system may beconfigured in this first mode, for example, at times of peak powerdemand on the grid, for example between 7 AM and 7 PM on weekdays.

In this first mode depicted in FIG. 7A, compressed air is flowed fromstorage unit 784 to compressor-expander 782 which is functioning as anexpander. Switch 788 is configured to allow thermal communicationbetween heat source 790 and heat exchanger 786 and/or storage unit 784.

As a result of the contribution of heat from the heat source in thismode, air expanding in the compressor-expander experiences a reducedchange in temperature, thereby producing an increased power output. Thispower output is in turn communicated through linkage 796 tomotor-generator 794 that is functioning as a generator. Power outputfrom the motor-generator may in turn be fed onto the power grid 798 forconsumption.

In a second mode of operation, system 780 is configured to replenish thesupply of compressed air in the storage tank. The system may beconfigured in this second mode, for example, at times of reduced demandfor power on the power grid.

In this second mode shown in FIG. 7B, motor-generator 794 receives powerfrom the power grid 798 (or directly from another source such as a windturbine or solar energy harvesting unit), and actuates linkage tooperate compressor-expander 782 as a compressor. Switch 788 isconfigured to allow thermal communication between heat sink 792 and heatexchanger 786 and/or storage unit 784.

As a result of the transfer of heat from the compressor-expander to theto the heat sink in this mode, air being compressed in thecompressor-expander experiences a reduced change in temperature, therebyresulting in a lower energy loss upon its conversion into compressedair. The compressed air is in turn communicated from thecompressor-expander to the compressed air storage unit 784, for laterrecovery in the first mode.

In certain embodiments, switch 788 may be temporal in nature, such thatit operates according to the passage of time. An example of this wouldbe the diurnal cycle, wherein during the day the heat exchanger and/orstorage unit are in thermal communication with the sun as a heat source.Conversely, at night the heat exchanger and/or storage unit would be inthermal communication with the cooling atmosphere as a heat sink. Insuch embodiments, the magnitude of the heat source could be amplified bytechniques such as reflection onto the heat exchanger and/or storagetank, or by providing the heat exchanger and/or storage tank with acoating configured to enhance absorption of solar radiation.

In certain embodiments, switch 788 may be physical in nature, such thatit is actuable to allow warm fluid from the heat source to be inproximity with the heat exchanger and/or storage unit, or to allow coolfluid from the heat sink to be in proximity with the heat exchangerand/or storage unit. Examples of this type of configuration include aswitch that is in selectively in fluid communication with pipes leadingto a power plant as the heat source, or to a body of water (such as acooling tower, lake, or the ocean) as the heat sink.

Operation of the various embodiments of systems described above, can beenhanced utilizing one or more techniques employed alone or incombination. One such technique is the introduction of a liquid into theair as it is expanding or being compressed. Specifically where theliquid exhibits a greater heat capacity than the air, the transfer ofheat from compressing air, and the transfer of heat to expanding air,would be improved. This greater heat transfer would in turn allow thetemperature of the compressing or expanding air to remain more constant.Such introduction of liquid during compression and expansion isdiscussed in detail in the '487 Application.

In certain embodiments, the liquid is introduced as a mist through aspray device. In other embodiments, the gas may be introduced bybubbling through a liquid. Other embodiments may employ both misting andbubbling, and/or multiple stages (see below) which employ misting and/orbubbling only in certain stages. We compute, during operation—and adjustas necessary—the volume of liquid spray required to maintain the ΔT ofcompression or expansion at the desired level.

Another technique which may employed to enhance operation of the system,is precise control over gas flows within the compressor-expander. Suchprecise control may be achieved utilizing a controller or processor thatis configured to be in electronic communication with various elements ofthe compressor-expander.

As described in detail above, embodiments of systems and methods forstoring and recovering energy according to the present invention areparticularly suited for implementation in conjunction with a hostcomputer including a processor and a computer-readable storage medium.Such a processor and computer-readable storage medium may be embedded inthe apparatus, and/or may be controlled or monitored through externalinput/output devices. FIG. 8 is a simplified diagram of a computingdevice for processing information according to an embodiment of thepresent invention. This diagram is merely an example, which should notlimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.Embodiments according to the present invention can be implemented in asingle application program such as a browser, or can be implemented asmultiple programs in a distributed computing environment, such as aworkstation, personal computer or a remote terminal in a client serverrelationship.

FIG. 8 shows computer system 810 including display device 820, displayscreen 830, cabinet 840, keyboard 850, and mouse 870. Mouse 870 andkeyboard 850 are representative “user input devices.” Mouse 870 includesbuttons 880 for selection of buttons on a graphical user interfacedevice. Other examples of user input devices are a touch screen, lightpen, track ball, data glove, microphone, and so forth. FIG. 8 isrepresentative of but one type of system for embodying the presentinvention. It will be readily apparent to one of ordinary skill in theart that many system types and configurations are suitable for use inconjunction with the present invention. In a preferred embodiment,computer system 810 includes a Pentium™ class based computer, runningWindows™ XP™ or Windows 7™ operating system by Microsoft Corporation.However, the apparatus is easily adapted to other operating systems andarchitectures by those of ordinary skill in the art without departingfrom the scope of the present invention.

As noted, mouse 870 can have one or more buttons such as buttons 880.Cabinet 840 houses familiar computer components such as disk drives, aprocessor, storage device, etc. Storage devices include, but are notlimited to, disk drives, magnetic tape, solid-state memory, bubblememory, etc. Cabinet 840 can include additional hardware such asinput/output (I/O) interface cards for connecting computer system 810 toexternal devices external storage, other computers or additionalperipherals, further described below.

FIG. 8A is an illustration of basic subsystems in computer system 810 ofFIG. 8. This diagram is merely an illustration and should not limit thescope of the claims herein. One of ordinary skill in the art willrecognize other variations, modifications, and alternatives. In certainembodiments, the subsystems are interconnected via a system bus 875.Additional subsystems such as a printer 874, keyboard 878, fixed disk879, monitor 876, which is coupled to display adapter 882, and othersare shown. Peripherals and input/output (I/O) devices, which couple toI/O controller 871, can be connected to the computer system by anynumber of approaches known in the art, such as serial port 877. Forexample, serial port 877 can be used to connect the computer system to amodem 881, which in turn connects to a wide area network such as theInternet, a mouse input device, or a scanner. The interconnection viasystem bus allows central processor 873 to communicate with eachsubsystem and to control the execution of instructions from systemmemory 872 or the fixed disk 879, as well as the exchange of informationbetween subsystems. Other arrangements of subsystems andinterconnections are readily achievable by those of ordinary skill inthe art. System memory, and the fixed disk are examples of tangiblemedia for storage of computer programs, other types of tangible mediainclude floppy disks, removable hard disks, optical storage media suchas CD-ROMS and bar codes, and semiconductor memories such as flashmemory, read-only-memories (ROM), and battery backed memory.

FIG. 9 is a schematic diagram showing the relationship between theprocessor/controller, and the various inputs received, functionsperformed, and outputs produced by the processor controller. Asindicated, the processor may control various operational properties ofthe apparatus, based upon one or more inputs.

An example of such an operational parameter that may be controlled isthe timing of opening and closing of a valve allowing the inlet of airto the cylinder during an expansion cycle. FIGS. 10A-C is a simplifiedand enlarged view of the cylinder 622 of the single-stage system of FIG.6, undergoing an expansion cycle as described previously.

Specifically, during step 2 of the expansion cycle, a pre-determinedamount of air V₀, is added to the chamber from the pressure cell, byopening valve 637 for a controlled interval of time. This amount of airV₀ is calculated such that when the piston reaches the end of theexpansion stroke, a desired pressure within the chamber will beachieved.

In certain cases, this desired pressure will approximately equal that ofthe next lower pressure stage, or atmospheric pressure if the stage isthe lowest pressure stage or is the only stage. Thus at the end of theexpansion stroke, the energy in the initial air volume V₀ has been fullyexpended, and little or no energy is wasted in moving that expanded airto the next lower pressure stage.

To achieve this goal, valve 637 is opened only for so long as to allowthe desired amount of air (V₀) to enter the chamber, and thereafter insteps 3-4 (FIGS. 10B-C), valve 637 is maintained closed. In certainembodiments, the desired pressure within the chamber may be within 1psi, within 5 psi, within 10 psi, or within 20 psi of the pressure ofthe next lower stage.

In other embodiments, the controller/processor may control valve 637 tocause it to admit an initial volume of air that is greater than V₀. Suchinstructions may be given, for example, when greater power is desiredfrom a given expansion cycle, at the expense of efficiency of energyrecovery.

Timing of opening and closing of valves may also be carefully controlledduring compression. For example, as shown in FIGS. 10D-E, in the steps 2and 3 of the table corresponding to the addition of mist andcompression, the valve 638 between the cylinder device and the pressurecell remains closed, and pressure builds up within the cylinder.

In conventional compressor apparatuses, accumulated compressed air iscontained within the vessel by a check valve, that is designed tomechanically open in response to a threshold pressure. Such use of theenergy of the compressed air to actuate a check valve, detracts from theefficiency of recovery of energy from the air for performing usefulwork.

By contrast, as shown in FIG. 10F, embodiments of the present inventionmay utilize the controller/processor to precisely open valve 638 underthe desired conditions, for example where the built-up pressure in thecylinder exceeds the pressure in the pressure cell by a certain amount.In this manner, energy from the compressed air within the cylinder isnot consumed by the valve opening process, and efficiency of energyrecovery is enhanced. Embodiments of valve types that may be subject tocontrol to allow compressed air to flow out of a cylinder include butare not limited to pilot valves, cam-operated poppet valves, rotaryvalves, hydraulically actuated valves, and electronically actuatedvalves.

Another example of a system parameter that can be controlled by theprocessor, is the amount of liquid introduced into the chamber. Basedupon one or more values such as pressure, humidity, calculatedefficiency, and others, an amount of liquid that is introduced into thechamber during compression or expansion, can be carefully controlled tomaintain efficiency of operation. For example, where an amount of airgreater than V₀ is inlet into the chamber during an expansion cycle,additional liquid may need to be introduced in order to maintain thetemperature of that expanding air within a desired temperature range.

1. An energy storage system comprising: a wind turbine; a gas compressorconfigured to be operated by the wind turbine through a linkage, the gascompressor comprising an element to effect gas-liquid heat exchange withcompressed gas, wherein the gas compressor comprises, a chamber definedwithin at least one wall, valving allowing selective fluid communicationbetween the chamber and the compressed gas storage unit, and a moveablemember received within the chamber to be selectively driven by the windturbine to compress gas within the chamber; a compressed gas storageunit in fluid communication with the gas compressor, and configured tostore gas compressed by the gas compressor; and a generator configuredto generate electrical power from expansion of compressed gas flowedfrom the compressed gas storage unit, wherein the moveable member isselectively configurable to transmit out of the chamber to thegenerator, a power of the compressed gas flowed from the compressed gasstorage unit expanding within the chamber; and the element is configuredto effect gas-liquid heat exchange with the compressed gas flowed fromthe compressed gas storage unit expanding within the chamber in anabsence of combustion.
 2. The system of claim 1 further comprising anacelle in rotational communication with a support structure through ajoint, the nacelle housing the wind turbine, the generator, and anexpander in fluid communication with the compressed gas storage unit andin physical communication with the generator.
 3. The system of claim 1wherein the linkage comprises a mechanical linkage.
 4. The system ofclaim 3 wherein the mechanical linkage comprises a gear.
 5. The systemof claim 1 wherein the linkage comprises a hydraulic linkage.
 6. Thesystem of claim 1 wherein the linkage comprises a pneumatic linkage. 7.The system of claim 3 wherein the mechanical linkage comprises arotating shaft.
 8. The system of claim 3 wherein the mechanical linkagecomprises a crankshaft.
 9. The system of claim 1 wherein the moveablemember is configured to reciprocate within the chamber.
 10. The systemof claim 1 wherein the movable member is configured to rotate within thechamber.
 11. An energy storage system comprising: a wind turbine; a gascompressor configured to be operated by the wind turbine through alinkage, the gas compressor comprising an element to effect gas-liquidheat exchange with compressed gas; compressed gas storage unit in fluidcommunication with the gas compressor, and configured to store gascompressed by the gas compressor; a generator configured to generateelectrical power from expansion of compressed gas flowed from thecompressed gas storage unit; a gas expander comprising a chamber definedwithin at least one wall; valving allowing selective fluid communicationbetween the chamber and the compressed gas storage unit; a moveablemember received within the chamber and selectively configurable totransmit out of the chamber, a power of the compressed gas flowed fromthe compressed gas storage unit expanding within the chamber; and asecond element configured to effect gas-liquid heat exchange with thecompressed gas flowed from the compressed gas storage unit expandingwithin the chamber in an absence of combustion.
 12. The system of claim11 wherein the gas expander comprises a dedicated expander incommunication with the generator.
 13. The system of claim 11 wherein thegas expander comprises a reversible expander/compressor such that themoveable member is configured to be selectively driven to flowcompressed gas to the compressed gas storage unit.
 14. The system ofclaim 11 wherein the moveable member is configured to reciprocate withinthe chamber.
 15. The system of claim 14 wherein the moveable membercomprises a solid piston.
 16. The system of claim 14 wherein themoveable member comprises a hydraulic liquid.
 17. The system of claim 11wherein the movable member is configured to rotate within the chamber.18. The system of claim 11 wherein the movable member is incommunication with the generator through a rotating shaft.
 19. Thesystem of claim 18 further comprising a control unit configured to:receive an input comprising a torque of the rotating shaft; calculate anoutput power from the input; and based upon the output power, control atiming of opening of a valve admitting the compressed gas flowed fromthe compressed gas storage unit.
 20. The system of claim 11 furthercomprising a control unit configured to: receive an input comprising atemperature in the chamber; calculate an efficiency from the input; andbased upon the efficiency, control a timing of opening of a valveadmitting the compressed gas flowed from the compressed gas storageunit.